Because all of their cells are surrounded by/very close to the environment in which they live. Diffusion will supply enough oxygen and nutrients to survive.

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What are the three main factors that affect the need for a transport system?

Size, surface area to volume ratio and level of activity.

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Why does size affect the need for a transport system?

Oxygen and nutrients cannot be diffused through multiple layers of cells and won't reach deep enough into the body.

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What effect does surface area to volume ratio have on the need for a transport system?

Small animals have a large surface area to volume ratio.This ratio is affected by animal's shape.Surface area to volume ratio of a large animal is small. Larger animal's surface area is not large enough to supply all oxygen+nutrients needed by cells.

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How are level of activity and transport systems linked?

Releasing energy from food by respiration requires oxygen. If an animal is very active, its cells need large supply of nutrients and oxygen to supply energy for movement.

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What are the features of a good transport system?

A fluid or medium to carry nutrients and oxygen around the body (this is the blood). A pump to create pressure to push the fluid around the body (this is the heart). Exchange surfaces to enable oxygen + nutrients to enter and leave blood when needed.

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Features of an efficient transport system

Tubes/vessels to carry the blood. Two circuits - one to pick up oxygen and another to deliver oxygen to the tissues.

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Describe single circulatory systems

Fish have a single circulatory system. Blood flows from the heart to the gills and then on to the body before returning to the heart.

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Describe double circulatory systems

E.g. mammals have these. One circuit carries blood to the lungs to pick up oxygen (pulmonary circulation). The other circuit carries oxygen and nutrients around the body to the tissues (systemic circulation). Blood flows through the heart twice.

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What happens in a single circulatory system?

Blood pressure is reduced as blood passes through tiny capillaries of gills. This means it will not flow quickly to the rest of the body which limits the rate at which oxygen and nutrients are delivered to respiring tissues. Efficient for fish.

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Describe what happens in a double circulatory system.

Heart increases blood pressure after passing through lungs so blood flows more quickly to body tissue.Systemic circulation carries blood at higher pressure than pulmonary circulation.Blood pressure can't be high in pulmonary, may damage capillaries.